The present invention relates generally to hydraulic control systems that form part of a vehicle such as an agricultural tractor. More particularly, the present invention relates to a control system that incorporates a pressure-flow compensating pump for delivering pressurized fluid to a plurality of fluid operated devices with a priority flow control valve incorporated therein that establishes priority of flow to a first fluid operated device whenever the pressure or flow requirements for that device are below a certain level.
Hydraulic control systems for controlling various functions that form part of a vehicle such as a tractor have been in existence for many years. Usually the hydraulic control system incorporates one pump for supplying pressurized fluid to a plurality of devices. Because of operational and safety reasons, it becomes necessary for the control system to incorporate some type of priority flow control to some of the devices. For example, in a tractor which incorporates power steering and power brakes, it is essential that at least one of these functions have priority for any flow that is delivered from the pump so that the operator is at all times in control of the vehicle.
In the past, the hydraulic control systems have normally been of either the open-center system or the closed-center system. The open-center system incorporates a fixed displacement pump that is capable of delivering a maximum given flow required for the entire system and a main control valve associated therewith connects the pump directly to the reservoir when the valve is in the neutral position. As expected in a system of this type, the pump is constantly delivering full flow at low pressure when the control valve is in a neutral position which results in considerable energy loss.
Furthermore, in most hydraulic control systems of this type, the system is designed to provide a maximum flow rate that is capable of operating all of the fluid operated devices simultaneously. As can be appreciated, the maximum flow rate for the pump is seldom needed when operating a vehicle of the type under consideration. Thus, in most instances, at least a portion of the power is wasted since the system seldom demands the maximum flow of the pump and the excess flow is bypassed through a pressure relief valve which means that further energy is lost.
The closed-center system incorporates a variable displacement, pressure compensated pump capable of a given maximum flow rate and the system is always operated at a predetermined maximum pressure. In this system, power is again wasted when less than the full pressure is required for operating the devices to which pressurized fluid is being supplied. In this system, the pump is constantly operating at full pressure which may result in heating of the fluid when the system is in a neutral position for extended periods of time.
Quite recently a third type of system has been developed which may be termed as a pressure-flow compensating or "load-sensing" hydraulic system. This type of system is generally disclosed in U.S. Pat. No. 3,486,334. The hydraulic circuit disclosed in this patent consists of a variable displacement pump, the output of which is controlled by a pressure compensating member that is connected to the unit being controlled so that the pump produces the required flow at the required pressure for delivery to the controlled unit. This type of hydraulic circuit delivers low flow at low pressure for minimum pump wear, flow losses and energy losses when the main control valves associated therewith are in a neutral position. The pressure-flow compensated hydraulic system has the unique advantage of being capable of maintaining high pressures without delivering a large volume of fluid and also delivering large volumes of fluid at lower pressures as per application needs. In other words, this system combines the advantages of the open-center system and the constant-pressure closed-center system while avoiding the major disadvantages.
While the pressure-flow compensating hydraulic system has numerous advantages over the other systems described above, to date such system has not found any widespread acceptance. At least one of the reasons for the non-acceptance of such a system is the fact that in the past, more expensive control valves and directional valves were required in the hydraulic circuit utilizing the pressure-flow compensating pump.